High-frequency electron discharge device of the triode type



Oct. 19, 1948. G SMITH 2,451,517 HIGH-FREQUENCY ELEcTRoN DISCHARGE DEVICE oF THE T RIoDE TYPE Filed Sept. 22,' 19`45 F/ G. E 7 9 /4 l ==F 6'\ A /A/L/fA/Taf'.

Patented Oct. i9, 1948 HIGH-FREQUNCY ELECTRON *DISCHARGE DEVICE FTHE T-RIODE TYPE Charles G. Smith, `Medford, .Mass., .assignor :to Raytheon Manufacturing Companyg Newton, Mass., a-corporation of Delaware Application September 22, 1943,Serial No. 503,354

4 lClaims.

This invention relates to an electronvdischarge device, and more particularly to a three-element tube for detecting or amplifying radiofrequencies,

especially ultra high frequencies.

Difiiculty has heretofore been experienced in attempts to use a three-element thermionic tube in the detection or amplification of frequencies of the'order of 1000 megacycles or more. The gate action of the grid becomes subordinate to other effects. For example, the mass of the electrons hinders the use of the gate action of the 'grid for these purposes.

' The device of the present invention is based Vupon different principles than any heretofore applied to this problem in that it utilizes a change `in the work function of an emission surface brought about by a change in the voltage gradient at the cathode. Thus while prior art devices have sought to control either (l) the number of electrons that leave the region near the emitter or- (2) the path of the electrons after they leave ithe emitter, the present invention seeksto control Vthe emission itself by controlling the work function of the emitting surface.

The change in work function may be expressed by the following equation:

where W is the work function in volts, and E is 'the gradient in volts per centimeter in front of the cathode.

Ordinarily the voltage gradient E' is small and accordingly the change in work function is not significant. The invention contemplates means whereby the voltage gradient is made large enough to affect the work function even with a small total voltage between the grid and the anode. To this end I provide a cathode having a sharp emission element in the form of either a point or an edge whereby the magnitude of the vpotential gradient at the cathode surface is made large and the change in the work function becomes significant.

The invention will be made fully apparent to those skilled in the art by reference to the following detailed description taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a cross-sectional view of a tube con'- structed in accordance with the present invention; and

Fig. 2 is a similar view of a modified form of the invention.

I Referring to -the drawing, and first to Fig. 1 lthereof, reference numeral l indicates a sealed highly evacuatedenvelope which may contain a 2 small amount of caseium for reasons which -will subsequently appear. Within the envelope 1I is an anode 2, a cathode 3 having a sharp emission element 4, and a grid 5 positioned adjacent said sharp emission element 4. The cathode 3and the grid 5 are connected by conductors 46 and 1 'to-a source of high frequency oscillations not shown. The anode 2 is connected to the positive terminal of a battery 8, or other source of biasing potential, by way of an output impedance or'load 9. The cathode 3 and the grid 5 are connected through high resistances Il) andi l `to a conductor i12 leading to a source of potential l1 which is negative relative to the anode 2 and which lmay be the negative terminal of the battery f8. The emission element 4 of the cathode 3 is heated by a filament I3. n

In the operation of this form of the'invention a positive bias of the order of volts vis applied -to the anode 2 from a, suitable source, such as the battery 8. It will be understood that this bias may be more or less than the 100 volts indicated depending upon the sharpness of the emitting point or edge 4 and the spacing and arrangement of the electrodes. The cathode point or edge 4i's heated to a temperature at which it emits electrons copiously, for example, of the order of 1000 degrees K. Preferably a magnetic field is imposed perpendicularly to the lines joining the point or potential of grid 5 causes an instantaneous change in the voltage gradient at the point or edge 4 and a corresponding change in the work function of Vthe cathode surface, and thus an instantaneous change in the electronic emission.

In order that the Variations in the potential df thegrid 5 may have an appreciable effect on the voltage gradient at the cathode surface and vthus effect a corresponding change in the work function of this surface, the voltage gradient at the cathode must be high, for example, of the order of 10,000 volts, but preferably higher. -In the embodiment shown, the high voltage gradient'at the cathode is secured by concentrating 4the electrical field to a point or a line at the cathode-surface. Such concentration is obtained in this instance by providing a cathode emitting surface in the form of a sharp point oredge.

Yvoltage gradient at the point or edge The point or edge forming the emitter should be sharp and its emission characteristics should be maintained in operation. For this purpose, I prefer to use an oxidized tantalum blade or point, and also provide a small quantity of caseium vapor in the tube. Small quantities of caseium settling upon the point or edge 4 tend to maintain its efficiency as an emitter. In some instances the point or edge may be formed of a fine wire.

With a high voltage gradient at the cathode surface obtained in the above described manner or in any suitable manner, the voltage swing of the grid 5 can change the gradient at the cathode surface to appreciably affect the work function of the cathode surface and thereby alter the electron emission thereof. This action is instantaneous since the change in the grid voltage instantly alters the voltage gradient at the cathode surface which instantly alters the work function of the surface.

In the form of the invention shown in Fig. 2, Where the same reference numerals as applied to Fig. l are applied to corresponding parts, the grid '5 is positioned between the cathode 3 and the anode 2 and closely adjacent to the sharp emission element 4. An orifice l5 is provided in the grid 5 immediately adjacent the point or edge 4. The input oscillations supplied by way of conductors 6 and 'I are in this instance applied to the grid 5 and the anode 2, and a resistance I6 for limiting the grid current is provided therebetween. The output impedance or load 9 is connected, as before, between the anode 2 and the positive terminal of the battery 8, the negative terminal of which is connected to the cathode 3. In this instance the coil Id is arranged to impose a magnetic eld parallel to the shortest line joining the point or edge il and the nearest point on the anode 2.

In the operation of this form of the invention, assuming that a positive bias is applied to the anode 2 and the cathode is heated, as in the previous form, a change in the potential of the grid 5 causes an instantaneous change in the This causes an instantaneous change in the work function of the cathode surface and thus an instantaneous change in electronic emission.

As in the form previously described, in order that the variations in the potential of the grid 5 may have an appreciable effect on the voltage gradient at the cathode surface and thus effect a corresponding change in the work function of this surface, the voltage gradient at the cathode surface must be high. Under these conditions the electronic emission of the cathode changes instantly with the voltage swing of the grid 5. The emitted electrons are highly accelerated by the high voltage gradient at the cathode surface and a-re thus projected through the opening l5 and are drawn to the anode 2. The imposition of the magnetic field parallel with the shortest line between the cathode point or edge 4 and the anode 2 aids in confining the electron ow in the direction of the anode 2 and in preventing deflection of the electrons in the direction of the grid 5.

In both of the embodiments a voltage swing of the grid, which may be of the order of 10-5 volt, can change the voltage gradient at the sharp edge to a degree sufficient to change the Work function of the cathode surface and thereby effect a change in the electronic emission of the surface. These effects are instantaneous with the change in the grid voltage and accordingly the current through the plate circuit, including the load impedance or the load 9, varies with the potential variations impressed upon the grid 5. Since the changes in the current through the tube are instantaneous and independent of the electron transit time, the invention is particularly adapted to the detection of ultra high frequency oscillations. Also, since small changes in the potential of the grid have the eect of redistributing the potential drop between the cathode and anode so that the voltage gradient at the cathode surface is greatly altered, a substantial amplification may be obtained.

By the term sharp, as used herein, I mean that the active surface of the cathode is so restricted as to give the desired concentration of the voltage gradient at the cathode surface. In order to preserve this concentration the active cathode surface should not be shielded by other portions thereof against this concentration. Thus, in the forms of the invention shown, the remaining portions of the cathode slope away rapidly from the active surface at the point or edge and are thus sufficiently spaced from the anode face with which the active surface cooperates so as not to interfere with su-ch concentration.

While I have disclosed the invention in conjunction with two embodiments thereof, other embodiments within the scope of the appended claims will be obvious to those skilled in the art from a consideration of the two embodiments shown. For example, in either of the embodiments the emission element shown may be either a point or a knife edge. In the form shown in Fig. 2, the opening through the grid will correspond in length to the length of the emission element regardless of the geometry of this element. Obviously other forms of cathode heating may be utilized. For example, the emission element may be directly heated rather than indirectly heated, as shown. The device may be used with or without the magnetic field, such elds being helpful in directing the electron flow and not an essential requirement. Also numerous other arrangements of the electrodes will readily suggest themselves from the foregoing description.

What is claimed is:

l. An electronic discharge device comprising an envelope evacuated to a degree sufficient to prevent gaseous conduction and containing an anode, a cathode having an emission element sufficiently sharp to provide an electron emissive surface having a voltage gradient at said surface of at least 10,000 volts per centimeter when the potential drop between said anode and said cathode is of the order of volts, a control electrode adjacent said cathode for altering the voltage gradient at said surface to change the work function of said emissive surface and there- -by effect instantaneous changes in the electronic emission of said surface, and means adjacent the space between said cathode and anode for imposing a magnetic iield upon said space in a direction to deflect electrons emitted by said cathode away from said control electrode and toward said anode.

2. An electronic discharge device comprising an envelope evacuated to a degree sulicient to prevent gaseous conduction and containing an anode, a cathode having an emission element suiliciently sharp to provide an electron emissive surface having a voltage gradient at said surface of at least 10,000 volts per centimeter when the potential drop between said anode and cathode is of the order of 100 volts, a control electrode adjacent said cathode for altering the work function of said emissive surface and thereby eiect instantaneous changes in the electronic emission of said surface, said control electrode having a perforation positioned adjacent said emissive surface and between said surface and said anode, and means adjacent the space between said cathode and anode for imposing a magnetic eld upon said space in a direction to deflect electrons emitted by said cathode away from said control electrode and through said perforation in said control electrode toward said anode.

\3. An electron discharge device comprising an envelope evacuated to a degree sufficient to prevent gaseous conduction and containing an anode, a cathode having an emission element sufficiently sharp to provide anelectron emissive surface having a voltage gradient at said surface of at least 10,000 volts per centimeter when the potential drop between said anode and cathode is of the order of 100 voltsa control electrode adjacent said cathode for altering the voltage gradient at said surface to change the work function of said surface and thereby effect instantaneous changes in the electronic emission of said surface, and means adjacent the space between said cathode andsanode for imposing a magnetic field upon said space in a direction to alter the paths of electrons drawn toward said control electrode and concentrate them in the direction of said anode.

4. An electron discharge device comprising an envelope evacuated to a degree sulcient to prevent gaseous conduction and containing an anode, a cathode having an emission element sufficiently sharp to provide an electron emissive surface having a voltage gradient at said surface of at least 10,000 volts per centimeter when the potential drop between said ari-ode and cathode is of the order of 100 volts, a control electrode adjacent said cathode for altering the voltage gradient at said surface to change the work function of said surface and thereby effect instantaneous changes in the electronic emission -of said surface, said control electrode comprising a flat metal member having a single perforation, said control electrode being positioned adjacent said emissive surface with said perforation in alignment with said surface and said anode to permit the unobstructed flow of electrons from any point on said surface to any point on said anode, and means adjacent the space between said cathode and anode for imposing a magnetic field upon said space, said eld extending parallel to and centered on a line between said cathode and said anode through said per foration.

CHARLES G. SMITH.

REFERENCES CITED UNITED STATES PATENTS Name Date Lilienfeld Dec. 15, 1925 Du Mont Feb. 21, 1933 Mahl Nov. 26, 1940 Litton July 8, 1941 Penny Mar. 17, 1942 FOREIGN PATENTS Number Number 

